1. Field of the Invention
The invention concerns a process and apparatus for selecting a control channel in a mobile radio station of a radio transmission system having a plurality of base radio stations to each of which one or more control channels are allocated.
2. Description of the Prior Art
In a radio communication system (radio transmission system) known from Published West German Patent application DE-AS No. 27 33 503 the service area (the entire primary coverage area) is divided up into a large number of zones (radio zones) (cellular system) and each radio zone is allocated at least one control channel (organization channel) and several traffic channels. In a radio zone with a large number of mobile telephone subscribers (mobile radio stations) the transmitter/receiver in each mobile radio station and in the base radio station must access several control channels. To simplify the transmission and reception separation filter, the transmission and reception frequency bonds are each divided into an upper and a lower band and the mobile radio stations are divided up into at least two groups each of which uses one of the transmission and reception frequency bands. The division of the transmission and reception frequency bands into two bands is chosen so that both such groups of mobile radio stations have a common band. The control channels at least are located in this common band. A mobile radio station of a first group selects a control channel, e.g. for the establishment of connections. To this end the mobile radio station scans the control channels allocated to its group for one with a good signal-to-noise ratio. If the signal-to-noise ratio of the selected control channel deteriorates because of interference to the radio field propagation or for some other reason, then the mobile radio station, on receipt of a certain control channel code, can also use a control channel allocated to another group, e.g. the second group. This allocation of control channels of another group is only undertaken in a radio zone which has a small number of mobile radio stations.
In a radio transmission system with a large number of transmission channels, e.g. 1000 transmission channels, this produces the need to place the control channels in a narrow frequency band of the transmission and reception frequency bands. Otherwise the search by the mobile radio stations for a control channel would take a long time.
If the number of control channels in a radio zone belonging to that radio zone or the frequency and consequently the channel number of such a control channel changes, then the mobile radio stations in that zone should automatically and independently assign themselves to the thus formed new set of control channels. If, for example, a new control channel is allocated to the radio zone when one of its control channels fails, then everything possible should be done to prevent the already existing control channels and the new control channel from becoming temporarily overloaded in the short term by a large number of individual transfers of mobile radio stations.
The problem underlying the invention was to devise a process for selecting a control channel in the mobile radio station from among any number of control channels located in the frequency band of the transmission channels. This problem has been solved by the process defined by claim 1.